Field of the Invention
The present invention relates to a liquid crystal display device and a manufacturing method thereof.
Description of the Background Art
As a display device, there is known an active matrix display device including a large number of pixels that are arranged in a matrix. The active matrix display device includes, for each pixel, a thin film transistor (TFT), which is a thin film semiconductor element, so that each pixel may be driven independently. Conventionally, as a semiconductor of a channel layer of the TFT for display use, amorphous silicon (a-Si) is often used. In recent years, with an increasing demand for high definition, an oxide semiconductor with higher mobility is starting to be used instead of a-Si.
As the active matrix display device, a liquid crystal display device, which is a display device that uses liquid crystal, is widely used. This device generally includes a pair of transparent substrates that are adhered to each other, and a liquid crystal composition sealed between the pair. The pair of transparent substrates is formed from a TFT substrate and a counter substrate. The TFT substrate includes a TFT for each pixel. The counter substrate may be provided with a color filter.
Liquid crystal display devices are roughly categorized into that of a longitudinal electric field method and that of a lateral electric field method. According to the longitudinal electric field, a pixel electrode is provided to one of the pair of transparent substrates (for example, the TFT substrate), and a common electrode is provided to the other of the pair (for example, the counter substrate). According to the lateral electric field method, both the pixel electrode and the common electrode are provided on one of the pair of transparent substrates (for example, the TFT substrate). For example, according to a fringe field switching (FFS) method, which is a type of the lateral electric field method, these electrodes are disposed on different layers of the same substrate. According to the FFS method, low voltage driving is possible, and also, a wide viewing angle and high contrast are achieved, and moreover, transmittance is high and bright display is enabled.
Furthermore, the liquid crystal display devices may be categorized into a transmissive type, a reflective type, and a semi-transmissive type having both features of the transmissive type and the reflective type. The transmissive type includes transparent electrodes as a pixel electrode and a common electrode, and performs display by transmitting light from a backlight device. The reflective type includes a reflective electrode as at least one of the pixel electrode and the common electrode, and performs display by reflecting light from a front light device or external environment.
Generally, the liquid crystal display device includes a storage capacitance for each pixel. The storage capacitance is mainly used to prevent the voltage of a pixel electrode from being affected by feed-through voltage caused by a change in the voltage on a scanning line or a signal line in a storage period when the TFT is in an off state. If a storage capacitance that is sufficiently larger than a parasitic capacitance is not secured, a phenomenon called flicker or burn-in may be caused due to the influence of the feed-through voltage. This may result in reduced image quality of the display device.
The following contents are disclosed by the description of Japanese Patent Application Laid-Open No. 2-81029 (1990). In a liquid crystal display device, an auxiliary capacitance electrode is disposed facing a display electrode across a storage capacitance (auxiliary capacitance) insulating film. In the manufacturing method, a gate insulating film and the auxiliary capacitance insulating film are formed from the same insulating film. Then, a part of the insulating film corresponding to the auxiliary capacitance insulating film is etched to a predetermined film thickness. According to this manufacturing method, the same insulating film is formed into an optimum film thickness for each of the gate insulating film and the auxiliary capacitance insulating film. Accordingly, at a part that is used as the gate insulating film, a sufficient film thickness may be secured so that short between a gate metal film and a drain/source metal film thereabove is not caused. Moreover, at a part that is used as the auxiliary capacitance insulating film, higher capacitance and reduced film thickness may be achieved, and there is no need to secure a large overlapping area between an auxiliary capacitance electrode and a display transparent electrode. This reduces a part where light is shielded by the gate metal layer and increases the aperture ratio of the pixel for an element structure where the gate metal film is used at the same time as an auxiliary capacitance.
With the transmissive liquid crystal display device, a high aperture ratio is desired so as to efficiently use light from a backlight. According to the technology described in Japanese Patent Application Laid-Open No. 2-81029 (1990), one of the electrodes forming the storage capacitance is made of a metal material, and because the material is opaque, light is blocked. Accordingly, with this technology, the effect of increasing the aperture ratio while securing the storage capacitance may not be sufficient. Accordingly, Japanese Patent Application Laid-Open No. 8-179363 (1996) discloses a liquid crystal display device, of the longitudinal electric field method, including a storage capacitance electrode of a transparent conductive material such as tin-doped indium oxide (ITO), tin oxide, or indium oxide, for example.
According to the FFS method, which is a type of the lateral electric field method, the overlapping area of a pixel electrode and a common electrode is great in plan view, and thus, a relatively large storage capacitance is secured by these electrodes alone. However, in recent years, the pixel size is reduced due to an ultra high definition liquid crystal display device, and it is becoming difficult to secure an area where electrodes can be formed. Accordingly, also with the FFS method, it is becoming difficult to secure a sufficient storage capacitance by the pixel electrode and the common electrode alone. Thus, Japanese Patent Application Laid-Open No. 2009-058913 separately provides a transparent storage capacitance electrode, for example.
In the case of applying the technology described in Japanese Patent Application Laid-Open No. 8-179363 (1996) or Japanese Patent Application Laid-Open No. 2009-058913 to a manufacturing method of a general TFT substrate, a deposition step and a patterning step have to be added to form a storage capacitance electrode. This results in increased manufacturing cost.